Nutritional Roles and Value of Dietary Selenium
Informed consumers increasingly demand foods with benefits beyond simply nourishment, matched to lifestyles, individual preferences and to meet specific dietary requirements. This has spurred interest in foods containing high levels of vitamins and minerals. Nutritional modification to make such foods may be achieved several ways, but preferably by making desirable changes on-farm to directly improve the food without subsequent manipulations.
One mineral of interest is selenium, a trace element essential to growth and health in humans and animals generally. In the body, selenium is incorporated into proteins to make selenoproteins, which are active in cell detoxification, redox cycling, and antioxidant defence against cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism that contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins regulate thyroid function and play a role in the immune system. Except in extreme cases, selenium deficiency does not by itself cause illness. Rather, it can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses.
As for all essential minerals, diet is the source of selenium intake. Criteria in New Zealand and elsewhere define the selenium Dietary Reference Intake (RDI) or Recommended Daily Allowance (RDA) for healthy consumers of various ages. New Zealand and Australian RDIs were set by the Australian National Health and Medical Research Council, and currently are:
Population sub-groupadult menadult womenboysgirlstoddlersinfantsSelenium7060-7570602510-15RDI(μg/day)
It should be appreciated by those skilled in the art that different countries have differences in RDI/RDA levels and the information above is provided by way of example only.
The selenium nutrition status of New Zealanders tends to be below World Health Organisation recommendations, as a consequence of the country's geology that determines mineral content of soil, which affects mineral uptake by crops and ultimately selenium content of locally grown foods. In the UK, recent changes in farming and import practices have also caused a drop in the selenium nutrition status of the general population.
People consuming a balanced and ample diet can usually meet their minimum intake requirement. However old habits, dieting or cultural practices that restrict variety of food choice can severely limit access to good selenium sources. Thus many consumers are looking for ways to boost their intake of selenium with nutritional supplements, often via pills but increasingly by eating nutrient-enhanced foods.
Meat and milk from New Zealand livestock naturally contains selenium, albeit at low concentrations which reflect composition of the grazing diets. Usual levels are 50 μg selenium/kg (range 30-85 μg/kg) in fresh lean beef or lamb1, and 2-11 μg selenium/liter in whole milk2. The contribution made to a person's daily selenium requirement by eating a 100 g or 100 ml serving of “typical” meat or milk would be 1-6% and 1-20% for adults and toddlers, respectively. Obviously, higher concentrations of selenium would increase the benefit derived from 100 g or smaller food servings. 1 West J. Compilation of nutritional data for New Zealand beef and lamb in domestic and export trade. Project 96MZ 64/4.3. New Zealand Meat Research and Development Council, Wellington, NZ, 1996. Unpublished report.2 Grace N D, Ankenbauer K, Alexander A M, Marchant R M. Relationship between blood selenium concentration or glutathione peroxidase activity, and milk selenium concentrations in New Zealand dairy cows. NZ Veterinary Journal 49:24-28, 2001.
In these foods (and in staple grains as well) selenium exists primarily as the organic chemical form selenomethionine, an analogue of the amino acid methionine. Selenomethionine can be incorporated into body proteins in place of methionine, and in this way serves as selenium storage in organs and tissues. Selenomethionine derived from foods is considered to have high bioavailability, in that it is well absorbed and utilised.
Improving Food Nutrition by Enriching Selenium Content
Enhanced foods can be produced by adding a nutrient such as selenium exogenously i.e. at the factory after harvest, or endogenously by causing extra selenium to be grown into the food prior to harvest or slaughter. The advantage of the latter is that selenium is incorporated into the food in its natural chemical form, is delivered to the consumer in an unadulterated way, and post-harvest processing or concentrating steps are avoided.
One method known to endogenously enrich the selenium content of milk from ruminants is by adding selenium or selenomethionine to the animal feed, as described for example in the Patent abstracts TW0565432B, CN1094899A and CN1217153A. The feed additive may also contain sodium selenite and sodium selenate, two inorganic forms of selenium salts. Examples of sodium selenite feed supplements are described in at least the abstracts of Chinese Patent No. CN1452884A, CN1439284A and CN1452883A.
An alternative method of increasing selenium levels in the animal and consequently in the meat or milk produced is to administer the selenium supplement by subcutaneous or intramuscular injection. Parenteral administration, which refers to the form being taken into the body in a manner other than through the digestive tract, has several distinct advantages over supplemented feed sources, as for example:                Treatment by injection is suitable for livestock that are primarily grazed on open pastures and not normally fed any supplements at all (e.g. in New Zealand).        The cost of producing and administering an injection formulation may be less than the cost of producing and feeding-out supplemented feeds;        The amount of selenium administered can be controlled accurately from an injection but is less certain from oral feeds due to variations in feed manufacture and amounts eaten by different animals;        Where only selected members of a herd are to be dosed with selenium, injection provides easier handling than oral feeds as treated animals do not need to be separated or their food regulated in any different way to other animals.        Dose quantities of selenium via injection are lesser, as selenium is not lost due to feed waste and poor absorption by the animal. This reduces the amount of selenium lost to the ground and potentially leeched into water supplies, and so gains advantage where environmental restrictions are in place.        
Many injectable selenium supplements are on the market but none contain selenomethionine. Among these is Novartis Deposel®, a fluid formulation of micronised particles of barium selenate (an inorganic selenium salt) that is injected subcutaneously into livestock animals and provides a long lasting reservoir of selenium at the site of injection. Deposel® is registered for use in sheep and cattle to treat and prevent selenium deficiency. Its purpose is animal health maintenance with no claims for effect on selenium content in meat or milk.
At least 15 other injectable selenium supplements are registered in New Zealand as “parenteral nutrients”. All are animal health remedies containing inorganic sodium selenite or sodium selenate, and none claim to affect meat or milk composition. The abstract of Russian Patent No. RU2222192C2 describes an intramuscular injection method of sodium selenite used to increase the volume of milk production, however no description is made of altered selenium levels in the milk produced.
The inventors have tested injectable formulations of barium selenate and sodium selenate in livestock and found that the animals' selenium status (i.e. the concentration of selenium in blood) is increased by treatment, but little selenium carries-through into the animals' milk or meat. Those food products are not sufficiently enriched to be used directly as a functional or enhanced food for human consumption without further processing such as concentrating steps. In one study completed by the inventors1, a single subcutaneous injection of barium selenate was used to increase animal selenium blood status. The treatment raised the selenium concentration of milk over an entire lactation, but only by 2-fold (from 6 to 11 μg selenium/liter of milk). 1Grace N D, Lee J, Mills A R, Death A F. Influence of Se status on milk Se concentrations in dairy cows. NZ Journal of Agricultural Research 40:75-78, 1997.
In a second publication2, methods for altering the amount of selenium in animal products are identified that include variation of pasture selenium intake (feed manipulation) or use of long-acting animal injections or use of oral ruminal boluses. The article also noted the use of an injection containing distinct chemical forms of organic selenium to enrich the selenium content of meat. The formulation(s) used to achieve that increase in meat selenium content was not described. 2 Knowles SO et al. Adding Nutritional Value to Meat and Milk from Pasture-fed Livestock. NZ Veterinary Journal 52:342-351, 2004.
As parenteral administration of selenium to livestock has distinct advantages over supplemented feed sources (see bullet points above), it would therefore be advantageous to have an injectable formulation and suitable methods for its administration that could be used to significantly increase the concentration of selenium in milk or meat for the use and benefit of human consumers, for example, as a boost to consumers' dietary intake.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.